High-speed imaging

A localized energy deposition in a thin layer of liquid between two solid glass plates excites waves in the liquid, solids, and their interfaces. Of particular interest is an elastic surface wave (Rayleigh wave) on the liquid–solid interface that travels faster than the shock wave in the liquid. The surface deformation caused by the Rayleigh wave expands the layer of liquid, thereby locally reducing the pressure below the cavitation threshold. The created tension nucleates many cavitation bubbles, which later collapse due to the passage of the trailing shock wave in the liquid. Interestingly, the bubbles are not arranged homogeneously but on concentric rings centered on the location of the energy deposition. We explain the formation of the concentric rings with the interaction between neighboring bubbles. The fluid–structure interaction is modeled with a coupled finite volume solver that couples a multi-phase compressible fluid region (water and bubble gas) with an elastic solid (glass). We find that the nucleation of bubbles in such a geometry relaxes the tension in their immediate vicinity and thereby suppresses the growth of neighboring bubble nuclei. This idea is confirmed by a Rayleigh–Plesset model of a bubble driven by a far-field pressure obtained from the finite volume simulation. The observed ring patterns are thus the result of the successive activation of statistically distributed nucleation sites into explosively expanding cavitation bubbles in an axisymmetric geometry, whose strong interaction on short distances leads to a hindrance of bubble growth in radially distinct regions.

We reveal for the first time by experiments that within a narrow parameter regime, two cavitation bubbles with identical energy generated in anti-phase develop a supersonic jet. High-resolution numerical simulation shows a mechanism for jet amplification based on toroidal shock wave and bubble necking interaction. The micro-jet reaches velocities in excess of 1000 m/s. We demonstrate that potential flow approximation established for Worthington jets accurately predicts the evolution of the bubble gas-liquid interfaces.

The flow around a bubble, precipitously expanding in a thin gap between flat walls, was found to have a peculiar feature: distinct fingering occurs at the bubble wall, which was observed through the ultra-high speed optical visualization. The effect is attributed to the reversal of the flow within boundary layers, which provokes the growth of instabilities at the inflection point and, when the surface tension is low enough (the local Weber number is high enough), leads to the fingering. In this paper, we show the high speed recordings of the fingering and model the evolution of the radial velocity to quantitatively confirm feasibility of the proposed instability mechanism.

In this methods paper, we explore the capabilities of high-speed ultrasound imaging (USI) to study fast varying and complex multi-phase structures in liquids and soft materials. Specifically, we assess the advantages and the limitations of this imaging technique through three distinct experiments involving rapid dynamics: the fl ow induced by a liquid jet, the dissolution of sub-micron bubbles in water, and the propagation of shear waves in a soft elastic material. The phenomena were simultaneously characterized using optical microscopy and USI with bubbles as contrast agents. In water, we use compounded high-speed USI for tracking a multi-phase flow produced by a jetting bubble diving into a liquid pool at speeds around 20 m/s. These types of jets are produced by focusing a single laser pulse below the liquid surface. Upon breakup, they create a bubbly fl ow that exhibits high reflectivity to the ultrasound signal, enabling the visualization of the subsequent complex turbulent flow. In a second experiment, we demonstrate the potential of USI for recording the stability and diffusive shrinkage of micro- and nanobubbles in water that could not be optically resolved. Puncturing an elastic material with a liquid jet creates shear waves that can be utilized for elastography measurements. We analysed the shape and speed of shear waves produced by different types of jetting bubbles in industrial gelatin. The wave characteristics were simultaneously determined by implementing particle velocimetry in optical and ultrasound measurements. For the latter, we employed a novel method to create homogeneously distributed micro- and nanobubbles in gelatin by illuminating it with a collimated laser beam.

Cavitation bubbles collapsing near boundaries create liquid flow through their center of mass movement, the formation of liquid jets, and long living vorticities. Here, we demonstrate robust pumping of the liquid with a compact and simple geometry, the open end of a thin-walled circular capillary tube filled with liquid. We study the dynamics of the cavitation bubbles and report on the resultant microjet formation through experiments and simulations. In the experiments, the dynamics of laser-induced cavitation bubbles are captured with high-speed microscopy. Simulations show excellent agreement with the experiments. The jet flow pumps liquid flow toward the capillary opening. The simulation reveals that, in the current study range, both the non-dimensional inner diameter of the capillary and the non-dimensional stand-off distance show influences on the jet width, and only the non-dimensional stand-off distance affects the maximum jet velocities. The results demonstrate that the confinement of the bubble within the capillary alters the anisotropic pressure field around the bubble, leading to a more mild collapse.

In a study conducted over 10 years ago (Petkovsek and Dular, 2013) [1] we noticed that the thin metal sheet sustains less cavitation damage when it is attached to an acrylic glass (PMMA) than in the case when we attached it to quartz glass (SiO₂). The reason for this was not explored at the time.

In the present paper we present a systematic study of single cavitation bubble erosion of a thin aluminum foil, which was attached to either PMMA or SiO₂ plate. We show that the damage sustained on the foil attached to PMMA plate is significantly smaller regardless of the bubble collapse distance from the boundary. The result is surprising since one would expect the weak foil to be severely damaged regardless of the material it is attached to.

By femtosecond illumination and high-speed image acquisition we were able to capture the formation and progression of the shock waves, which are emitted at cavitation bubble collapse and observed that they are reflected on SiO₂ boundary but that they traverse the PMMA bulk material. We offer an explanation that to achieve less damage the bulk material needs to have acoustic impedance similar to the one of the liquid medium in which cavitation occurs.

Further on, we constructed a simple composite material where PMMA was attached to the SiO₂ and showed that we can partially mitigate the damage. This was further confirmed by ultrasonic cavitation erosion tests.

The results also imply that the cavitation damage originates solely from the shock wave, which is emitted at cavitation bubble collapse – consequently putting the idea of microjet impact mechanism under question. Finally, the study offers a new exciting approach to mitigate cavitation erosion by fine tuning the acoustic impedance of the coatings.